Roller type powder sprayers are commonly employed in printing operations for directing powder onto printed sheets prior to delivery to a stacking station in order to maintain sheet separation and prevent offset. Such roller type sprayers typically comprise a box-like, powder-containing housing having an elongated discharge opening in the bottom thereof within which a roller having a relieved outer surface is rotatably disposed. As the roller is rotated, doctor blades which define the discharge opening permit only a metered quantity of powder, as contained within the relieved areas of the roller, to be carried to a discharge zone outside the housing. Electrostatic means may be employed to facilitate complete removal of powder from the roller at the discharge zone.
Heretofore, problems have been incurred in maintaining a supply of powder to the sprayer housing. Conventionally, mechanically operated chains have been employed for conveying powder from a powder supply hopper to the sprayer housing. Such chains typically are directed through tubular members connecting the supply hopper and sprayer housing in order to guard against the discharge of the powder into the atmosphere. A drawback of such chain-type conveyers has been that they usually require the storage hopper to be located with the discharge end thereof substantially level with the sprayer housing. This necessitates the inlet of the supply hopper being located at such a high elevation as to make it cumbersome to refill. Moreover, powder tends to accumulate on the chain, and after prolonged use, can create clogged conditions within the tubular members communicating between the supply hopper and the sprayer housing so as to impede operation of the sprayer. Further operational problems occur if the sprayer housing overfills, including the undesirable discharge of powder into the environment around the sprayer.
In an effort to overcome such problems, proposals have been made for the pneumatic transfer of powder from the supply hopper to the sprayer housing, such as shown in application Ser. No. 07/241,040 assigned to the same assignee as the present application. Such system, however, requires filters for air exiting the system, which necessitates periodic filter replacement and/or cleaning and, if not properly effected, can permit escape of powder to the outside environment. The quantity of powder transferred between the hopper and the applicator roll in such pneumatic system also has been found to vary depending upon the level of powder in the hopper, and when low levels are present, considerable time can be required to fill the applicator roller housing. The pneumatic supply and return conduits connected between the supply hopper and applicator roll also still are susceptible to clogging, which can hinder efficient operation of the system. The system further requires timers and interlocks for controlling the fill cycle for the applicator roller, which are costly in design and subject to selected operational settings.